Sustainable Season Extension for Gardening - Considerations for Design; Gardening Guidebook

8/9/2019 Sustainable Season Extension for Gardening - Considerations for Design; Gardening Guidebook

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The National SustainableAgriculture Information Service,ATTRA (www.attra.ncat.org),was developed and is managedby the National Center forAppropriate Technology (NCAT).

The project is funded througha cooperative agreement withthe United States Departmentof Agricultures Rural Business-Cooperative Service. Visit theNCAT website (www.ncat.org/sarc_current.php) formore information onour other sustainableagriculture andenergy projects.

1-800-346-9140 www.attra.ncat.orgA project of the National Center for Appropriate Technology

By David Ryan, PE

NCAT Energy Engineer

and Tammy Hinman,

NCAT HorticultureSpecialist

Published September

2011

NCAT

IP416

Contents

Sustainable Season Extension:

Considerations for DesignGrowing food in cold climates can be diffi cult, but utilizing season-extension techniques, such as coldframes, hoop houses, and greenhouses, can help overcome these challenges. This publication assesses sev-

eral greenhouse design elements and their effectiveness at extending the growing season in cold climates.

Introduction ......................1

Methodology ....................2

Basic DesignPrinciples ............................2

Solar Orientation .............3

Glazing ................................3

Heat Loss ............................8

Heat Balance ...................11

Weather inButte, Montana ..............11

Heat Storage .................. 14

Conclusion ...................... 15

References ...................... 15

Further Resources ........ 15

Introduction

Growing food in cold climates canbe difficult. Extreme temperatureuctuations, short growing seasons, and

low humidity are among the challenges facedby growers in these climates. However, utilizingseason-extension techniques, such as cold frames,hoop houses, and greenhouses, can help over-come these challenges. Season extension refersto techniques or structures that allow crops tobe grown beyond their normal outdoor grow-ing season. Season extension enables growers toproduce cash crops in cold climates, improving

the economics of small farming operationsand increasing consumer access to locally

grown produce.Tis publication helps producers evaluate green-house design elements for cold climates. Te tech-niques described here could apply to cold frames,greenhouses, and hoop house structuresor toany building, for that matter. Tus, the termsgreenhouse and hoop house are used inter-changeably throughout this publication.

In small farm productionas in any otherindustryeconomics rule. High-cost com-mercial greenhouses are used in every state to

Hoop house structure in Belgrade, Montana. Photo: Dave Ryan, NCAT


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Page 2 ATTRA Sustainable Season Extension: Considerations for Design

MethodologyTis publication relies on ypical Meteorologi-cal Year (MY) data to help evaluate greenhousedesign in order to better predict the growing sea-son, with an overall goal of extending the seasonfurther into the fall and starting the season earlierin the spring. Te methods described could alsobe used to evaluate the growing season for plants

that need to be kept at higher temperatures bychanging the desired indoor temperature in thecalculations. Te examples are based on the freez-ing temperature point (32o F).

Specically, the evaluations in this publication arebased on MY data for Butte, Montana (USDAZone 3). Tis evaluation will consider the impli-cation of Buttes harsh climate in three cases:

An open garden with no greenhouse at all.

A base case hoop house that is 4meters by 8 meters (about 12 feet by24 feet). Tis model hoop house is con-structed using PVC pipe and is covered

with a single layer of 6-mil polyethyl-ene. Tis model has a high air-inltra-tion rate (often referred to as leaky).

A hoop house with Solexx coveringand built much tighter (less leaky),including rigid foam insulationaround the perimeter. Tis effi cientcase hoop house will demonstratethat the additional cost and effort in

making the hoop house more energy-effi cient will result in a longer cold-climate growing season.

Basic Design PrinciplesTe two primary goals when designing a hoophouse for cold climates are effi cient collectionof solar energy and prevention of heat loss.(Heat storage also is important, but since it isso dependent upon the individual greenhouse,it is only discussed in broad terms.) Tese goals

can be accomplished with the following basicdesign principles:

Solar Orientation:Te hoop house isoriented to receive maximum solar heatduring the winter.

Glazing: Glazing material is chosenand installed in a manner that mini-mizes heat loss.

produce cash crops. Generally, these green-houses are cooled and heated using electric fansand evaporative coolers and natural gas- or oil-red heaters. Te energy use in these buildingscan be 10 to 30% of the operating costs of thegreenhouse (Wisconsin Focus on Energy, 2008).Hoop houses (also called high tunnels) are sim-ple greenhouses that have little or no heating orcooling systems other than natural solar heatingand natural ventilation. Hoop houses provide lesscontrol of environmental conditions than fullgreenhouses, but they also cost substantially less.Hoop houses are constructed of pipes bent intoa hoop shape and typically have no foundation.Tey usually are covered with a single layer ofplastic and are ventilated by rolling up the cover-ing on the sides. A typical high tunnel is relativelyeasily taken down and put up. In most areas, thisis an advantage because it does not have a founda-tion and is considered a non-taxable structure.

his publication uses a common hoop housedesign as a basis for design comparisons. Te goalis to determine the extent to which certain designfeatures can extend the growing season. As youwill see, the length of the growing season in a coldclimate can be signicantly increased by using ahoop house and improving coverings, decreasingair leakage, and increasing insulation levels.

Solar Greenhouses

Specialty Crops for

Cold Climates

Season Extension

Techniques for Market

Gardeners

Related ATTRAPublicationswww.attra.ncat.org

Are you a cold-climate farmer?The United States Department of Agriculture has designated specificzones throughout the United States based on average first and last

frost-free dates and minimum and maximum temperatures. The USDAHardiness Zone Map divides North America into 11 separate zones;each zone is 10 Fahrenheit warmer (or colder) in an average winterthan the adjacent zone (U.S. National Arboretum, 1990). If you see ahardiness zone in a catalog or plant description, it likely refers to theUSDA map. Referring to the map helps establish a rough estimate ofwhat types of crops, trees, and shrubs grow in your area. To find yourzone, visit the National Arboretum website at www.usna.usda.gov/Hardzone/index.html.

Hoop house. Source: NCAT
http://www.usna.usda.gov/Hardzone/index.htmlhttp://www.usna.usda.gov/Hardzone/index.html


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Page 3ATTRAwww.attra.ncat.org

winter. Tis publication assumes a north-southorientation.

Shading

Te greenhouse should be sited to achieve the

greatest possible exposure to the sun. rees andother buildings shading the greenhouse will

have a dramatic negative effect on the plants

growing in the shaded area of the greenhouse.

In cold climates, the goal is to maximize the

solar resource with proper orientation. Coolerair temperatures can help provide cooling tothe greenhouse.

GlazingTe percentage of available solar energy enter-

ing the greenhouse is called the Solar Heat

Gain Coefficient (SHGC). SHGC varies by

material and is determined by the transmis-

sion of light through the glazing and the

heat absorbed by the glazing and re-radiated

into the greenhouse. he heavier the glazing

materials and construction, the more SHGCdiffers from light transmission, but SHGC is

generally larger than transmissionroughly

2%. Table 1identifies light transmission for

common glazing products.

Heat Loss:Large amounts of insulation

are installed where there is little or no

direct sunlight.

Solar OrientationSince the suns energy is strongest on the south-

ern side of a building, glazing for solar green-

houses should ideally face true south. How-

ever, if trees, mountains, or buildings blockthe path of the sun when the greenhouse is in

a true south orientation, an orientation within

15 to 20 of true south will provide about 90%

of the solar capture of a true-south orientation.

Te latitude of your location and the location

of potential obstructions may also require that

you adjust the orientation of your greenhouse

slightly from true south to obtain optimal solar

energy gain (Mother Earth News, 1978). Some

growers recommend orienting the greenhouse

somewhat to the southeast to get the best solargain in the spring, especially if the greenhouse

is used primarily to grow transplants (Wang,

2006). o determine the proper solar orienta-

tion in your area, visit the sun chart program at

the University of Oregon Solar Radiation Moni-toring Laboratory website at http://solardat.uore-gon.edu/SunChartProgram.html.You need to

know your latitude, longitude, and time zone to

use this program.

Slope of Glazing MaterialIn addition to north-south orientation, green-

house glazing should be properly sloped to

absorb the greatest amount of the suns heat. A

good rule of thumb is to add 10 or 15 to the

site latitude to get the proper angle. For exam-

ple, if you are in northern California or central

Illinois at latitude 40 north, then the glazing

should be sloped at a 50 to 55 angle (40 + 10

or 15) (Bellows, 2008).

Hoop houses are basically all glazing; the coveris placed over the hoops and pulled tight down

to the ground or near to the ground. Different

references conict on which orientation is the

best for hoop houses, with some indicating thata north-south orientation is best in the sum-

mer and an east-west orientation is best in the

Source: Barbara Bellows. 2008. Solar Greenhouses. ATTRA National Sustainable

Agriculture Information Ser vice

Figure 1. Solar Path at 40oNorth Latitude
http://solardat.uoregon.edu/SunChartProgram.htmlhttp://solardat.uoregon.edu/SunChartProgram.htmlhttp://solardat.uoregon.edu/SunChartProgram.htmlhttp://solardat.uoregon.edu/SunChartProgram.html


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Glasssingle layer

Light transmission*: 85-90%

R-value**: 0.9

Advantages:

Lifespan indefinite if not broken

Tempered glass is stronger and requires fewer support

bars

Disadvantages: Fragile, easily broken

May not withstand weight of snow

Requires numerous supports

Clear glass does not diffuse light

Factory-sealed double glass


R-value**: 1.5-2.0 for double layer; 2.5 for low-emissivity

(low-e)

Advantages:

Lifespan indefinite if not broken

Can be used in areas with freezing temperatures

Disadvantages: Heavy

Clear glass does not diffuse light

Diffi cult to install, requires precise framing

Polyethylenesingle layer

Light transmission*: 80-90% for new material

R-value**: 0.87 for single film

Advantages:

IR films have treatment to reduce heat loss

No-drop films are treated to resist condensation Treatment with ethyl vinyl acetate results in resistance to

cracking in the cold and tearing

Easy to install; precise framing not required

Lowest-cost glazing material

Disadvantages:

Easily torn

Cannot see through

UV-resistant polyethylene lasts only 1 to 2 years

Light transmission decreases over time

Expands and sags in warm weather, then shrinks in

cold weather

Polyethylenedouble layer


R-value** double films: 1.5 for 5-mil film; 1.7 for 6-mil film

Advantages:

Heat loss significantly reduced when a blower is used to

provide an air space between the two layers IR films have treatment to reduce heat loss

No-drop films are treated to resist condensation

Treatment with ethyl vinyl acetate results in resistance to

cracking in the cold and tearing

Easy to install; precise framing not required

Lowest-cost glazing material

Disadvantages:

Easily torn

Cannot see through

UV-resistant polyethylene lasts only 1 to 2 years


Expand and sag in warm weather, then shrink in cold

weather

Polyethylenecorrugated high-density


R-value**: 2.5-3.0

Advantages:

Mildew-, chemical-, and water-resistant

Does not yellow

Disadvantages:

n/a

Laminated acrylic/polyester lmdouble layer

Light transmission*: 87%

R-value**: 1.8

Advantages:

Combines weatherability of acrylic with high service

temperature of polyester

Can last 10 years or more

Disadvantages:

Acrylic glazings expand and contract considerably;

framing must allow for this change in size Not fire-resistant

Table 1. Glazing Characteristics


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Impact modied acrylicdouble layer


Advantages:

Not degraded or discolored by UV light

High impact strength; good for locations with hail

Disadvantages:

Acrylic glazings expand and contract considerably;

framing must allow for this change in size Not fire-resistant

Fiber reinforced plastic (FRP)

Light transmission*: 85-90% for new material

R-value**: 0.83 for single layer

Advantages:

The translucent nature of this material diffuses and

distributes light evenly

Tedlar-treated panels are resistant to weather, sunlight,

and acids Can last 5 to 20 years

Disadvantages:


Poor weather-resistance

Most ammable of rigid glazing materials

Insulation ability does not allow snow to melt

Polycarbonatedouble-wall rigid plastic


R-value**: 1.6 for 6mm plastic; 1.7 for 8mm plastic

Advantages:

Most fire-resistant of plastic glazing materials UV-resistant

Very strong

Lightweight

Easy to cut and install

Provides good performance for 7 to 10 years

Disadvantages:

Can be expensive

Not clear, translucent

Polycarbonate lmtriple- and quad-wall rigid plastic


R-value** triple walls: 2.0-2.1 for 8mm film;

2.5 for 16mm film

R-value** quad wall: 1.8 for 6mm film; 2.1 for 8mm filmAdvantages:

Most fire-resistant of plastic glazing materials

UV-resistant

Very strong

Lightweight

Easy to cut and install

Provides good performance for 7 to 10 years

Disadvantages:

Can be expensive

Not clear, translucent

* Framing decreases the amount of light that can pass through and be available as solar energy.** R-value is a common measure of insulation. The higher the R-value, the greater the materials insulating ability.

Table 1. Glazing Characteristicscontinued

A note on units of measure:Figure 2 (on page 6) shows average solar radiation available on an annual basis in kilowatt-

hours (kWh) per day. One kWh is equal to 3,413 British thermal Units (Btu). Most heating systems are rated in Btu per hour

of heating capacitya typical home-heating furnace might be 100,000 Btu per hour of capacity. According to the NREL

map, Butte, Montana, gets an average of about 4.5 kWh per square meter of surface tilted at the degrees of latitude (in

Butte, that is about 45o F). This calculates to an average of 4.5 kWh per day per square meter of surface * 3,413 Btu per

KWh = 15,359 Btu per day per square meter of input. If the surface of the greenhouse glazing at the latitude angle of tilt

is 4 meters by 8 meters (about 12 feet by 24 feet), then the average annual solar input is 15,359*4*8 = 491,488 Btu per day.

The other calculations in this publication use the horizontal illuminance of a hoop house with a mostly clear covering.

Source: Bellows, 2008


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of system types and congurations for one or

more locations.

About TMY DataA ypical Meteorological Year is not necessar-

ily a good indicator of conditions over the next

yearor even the next ve years. Rather, it rep-

resents conditions judged to be typical over along period of time, such as 30 years. Because

they represent typical rather than extreme

conditions, MYs are not suited for design-

ing systems and their components to meet the

worst-case conditions occurring at a location.

However, MY data indicates what can be

expected for typical weather conditions for a

location, such as the following:

Te energy gain in a greenhouse due to solar

heating is a function of how much solar radi-

ation falls on the surface of the greenhouse

covering, both directly and indirectly. Most of

the energy is directly from the sun, but reec-

tive light from surfaces such as buildings and

trees also can result in solar gain. While Figure

2shows annual average solar input, daily aver-age input is more useful because it helps iden-

tify the critical ends of the growing season.

Te goal is to determine how much energy is

typically available in the spring and fall in order

to plan how much season extension is achiev-

able. MY data provides a standard for hourly

data for solar radiation and other meteorological

elements that allows performance comparisons

Source: NREL

Figure 2. Annual PV Solar Radiation by Location


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available and a 1991-2005 period of record forall other locations (Wilcox, 2008).

Te user manual for MY3 data can be foundat www.nrel.gov/docs/fy07osti/41364.pdf, andthe MY3 data is located at http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.html.

MY data is presented hourly. Daily solar heat

gain and heat loss can be calculated simplyby adding the 24 hours of data for each day.Table 2provides an example of MY3 data forJanuary 1 for Butte, Montana. Te last column,GHI (Global Horizontal Illuminance), repre-sents direct solar incidence plus diffuse radiationcoming onto the horizontal surface from objectsaround the surface.

otal amount of direct and diffusesolar radiation received on a horizontalsurface during a period of time (tocalculate solar heat gain)

Dry bulb air temperature (tocalculate heat loss from the greenhouse)

Other weather factors also come into play, such

as wind speed and cloud cover, but this publica-tion analyzes solar gain and heat loss from thegreenhouse structure only.

he most current information can be foundin MY3 data. MY3 is based on data for 1,020locations in the United States, including Guam,Puerto Rico, and U.S. Virgin Islands, derivedfrom a 1976-2005 period of record where

Date

(MM/DD/YYYY) Time (HH:MM) ETR (W/m 2) ETRN (W/m^2) GHI (W/m^2)

1/1/2002 1:00 0 0 0

1/1/2002 2:00 0 0 0

1/1/2002 3:00 0 0 0

1/1/2002 4:00 0 0 0

1/1/2002 5:00 0 0 0

1/1/2002 6:00 0 0 0

1/1/2002 7:00 0 0 0

1/1/2002 8:00 0 0 0

1/1/2002 9:00 56 1097 6

1/1/2002 10:00 234 1415 35

1/1/2002 11:00 379 1415 69

1/1/2002 12:00 473 1415 89

1/1/2002 13:00 507 1415 116

1/1/2002 14:00 480 1415 181

1/1/2002 15:00 394 1415 155

1/1/2002 16:00 255 1415 186

1/1/2002 17:00 76 1262 25

1/1/2002 18:00 0 0 0

1/1/2002 19:00 0 0 0

1/1/2002 20:00 0 0 0

1/1/2002 21:00 0 0 0

1/1/2002 22:00 0 0 0

1/1/2002 23:00 0 0 0

1/1/2002 24:00 0 0 0

Table 2. Typical Meteorological Year Data for January 1 (Butte, Montana)

Source: NREL
http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.htmlhttp://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.htmlhttp://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.htmlhttp://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.html


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Figure 3uses Excel to show the typical solargain for Butte, Montana.

Heat LossTere are three primary causes of heat loss in agreenhouse:

Conduction heat loss from the coveringor glazing. Tis is caused by the tem-

perature difference between the insideof the greenhouse and the outside air.

Inltration, or leakage of cold air intothe greenhouse. Cold air entering thegreenhouse forces warm air out, requir-ing the cold incoming air to be heated.

Perimeter heat loss, or heat loss aroundthe outside margins of the greenhouseat ground level. Perimeter heat loss canbe reduced by insulating and sealing thegreenhouse to reduce air inltration.

Te sum of the days GHI is 862 watt-hoursper horizontal square meter. In January, 0.862kWh per square meter, or 94,144 Btu per day,for a 12-foot by 24-foot greenhouse will prob-ably not keep the greenhouse from freezingin the cold January weather. Table 6 later inthis publication compares the heat loss for thesame day in January with the solar input onthat day.

A Note on Adding Up the DataDay by Day

MY data is stored in a computer format called

ASCII ext. Te data can be used in its rawform, but because there are 8,760 hours in a yearand data for each hour, doing so would requireconsiderable data entry using a calculator. It ismore effi cient to use an Excel spreadsheet or,

even better, an Access database to analyze data.

Figure 3. Typical Solar Gain for Butte, Montana

Chart represents solar gain on the oor of the hoop house. Source: NCAT


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glazing material, the thickness of the air spacein between, and, in some cases, whether the airspace is lled with some gas other than air (suchas argon). Glazing manufacturers also may puta coating on the glazing material that decreasesthe emissivity of the glass (called low-e coat-ings). Te units on the overall heat transfer coef-cient are Btu per square foot per degree F perhour (BtuFt2F Hr).

Tis publication analyzes two theoretical green-houses of the same size and shape but withdifferent coverings, different air leakage rates,and different perimeter losses.

Conduction Heat Loss

Conduction heat loss (Qc) is a function of theheat-transfer characteristics of the greenhouse

covering, the area of the greenhouse covering,and the temperature difference across the green-house covering. It is calculated with the formulaQc = UAD. Te heat transfer coeffi cient (U) isthe reciprocal of the additive resistances of thecovering(s), the air space between the coverings,and the layers of air just on the inside and theoutside of the coverings. Air movement on theinside and outside of the greenhouse coveringalso contributes to conduction heat loss throughthe cover. Figure 4represents the transfer of heatthrough a double-layer greenhouse covering.

Most exible greenhouse coverings are 6 milsthick. A mill is 1/1000 of an inch thick, so6 mils is 0.006 inches thick. Te air space inbetween the coverings can be designed into agreenhouse. Te overall heat transfer coeffi cients(U-values) for various glazing materials are listedin able 3. U-value assumes some air movementand takes into account the thickness of the

Product Light Transmission (%) U- Value

3.5mm Solexxroll 75 0.476

6mm Polycarbonate clear 80 0.645

Bronze 42 0.645

Opal 44 0.645

Gray 42 0.645

8mm Polycarbonate clear 80 0.635Bronze 42 0.635

Opal 44 0.635

Gray 42 0.635

16mm Polycarbonate clear 78 0.545

Bronze 42 0.545

Opal 44 0.545

Gray 42 0.545

Single clear glass 90 1.11

Single bronze glass 68 1.11

Single azurlite glass 77 1.11

Double clear glass 82 .55

Double bronze glass 62 .55

Double azurlite glass 70 .55

Double clear glass with low-e 75 .35.

Double bronze with low-e 57 .35

Double SolarCool with low-e 24 .35

Double azurlite with low-e 65 .35

Double clear with low-e &

argon gas

76 .33

Double bronze with low-e &argon gas

57 .33

Double SolarCool with low-e

& argon gas

24 .33

Double azurlite with low-e &

argon gas

65 .33

Single polyethylene film 90 1.2

Table 3. Types of Cover Materials

Source: University of Georgia, College of Agricultural and Environmental Sciences

Source: NCAT

Figure 4. Transfer of Heat through aDouble-Layer Greenhouse Covering


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per square foot per degree F per hour* em-perature difference, or 706 for the single-roll covering, and 291.6 for the twin-wallSolexx. emperature difference is required tocomplete the calculation, which can be obtainedfrom MY3 data for typical outdoor air tem-peratures for our location. Again, for freezeprotection, greenhouse temperatures no lowerthan 32o F must be maintained.

Infiltration Heat Loss

Cold air (less than 32o F) leaking into the green-house must be heated up or it will cause freez-ing damage to the plants in the greenhouse. Teheat loss (Q

i) is basically the mass of air (Vol-

ume * density of air) moving into the green-house from the outside multiplied by the heatcapacity (C

p) of the air and then multiplied by

the temperature difference () between theoutside air and the inside air (Qi = VrC

p).

Again, we are only interested in the hours of theyear when the outside air is below freezing. Tevolume of air moving into the greenhouse dueto leakage is measured in the industry in typi-cal air changes per hour (ACH). An air changemeans the entire volume of air in the greenhousehas been replaced by outside air due to leak-age through the covering and around the baseof the greenhouse. A leaky greenhouse mighthave two ACH, while a very tightly built green-house might have half of an ACH. Our analysisincludes two cases: one in which the greenhousehas one ACH and one in which the greenhousehas two ACH.

For example, if a hoop house is 24 feet long andsemicircular in shape with a radius (r) of 6.37feet, the volume of air in the greenhouse is (r2/2)* l = 1,530 cubic feet of air. Te density of air isabout 0.071 pounds per cubic foot at the temper-atures we are concerned with, and the heat capac-ity of air is about 0.17 Btu per pound degree F.Tus, the heat loss from one air change per houris 486.9 cubic feet*0.071 pounds / cubic feet *

0.17 Btu / pounds F * = 18.46 . MY3 dataidenties the hours where the air temperature isless than 32o F to nd the length of the season.

Perimeter and Ground Heat Loss

Perimeter Heat LossHeat loss around the perimeter of the green-house can be calculated using Qp = F * P *

Covering (Glazing) Losses

Te area for heat transfer includes all of the sur-faces of the greenhouse exposed to the outside

air. Our example analysis assumes ground tem-

perature to be no less than 32o F (since the goalis freeze protection). Te analysis is based on

a hoop house that is constructed with 2-inch-

diameter PVC pipe and measures 12 feet wide

by 24 feet long (about 4 meters by 8 meters). TePVC pipe is 20 feet long, so calculating the area

of the covering is straightforward: 20 feet by 24

feet = 480 square feet. Te area of the ends of

the greenhouse if the ends are semicircles is r2,

where r = 6.37 feet. Te area of the ends togetheris 127.48 square feet. Te total area exposed to

the weather is 607.5 square feet.

Our analysis compares two types of covering

material: single-layer polyethylene, with a heat

transfer coeffi cient of 1.2; and Solexx roll cov-

ering, which has a U-value of 0.48. Te heat

transfer (Qc) is now 607.5 square feet * U in Btu

Greenhouse Covering R-Value U-Value

5mm Solexx Panels* 2.30 0.43

3.5mm SolexxPanels*

2.10 0.48

8mm Triple-WallPolycarbonate

2.00 0.50

Double-Pane StormWindows 2.00 0.50

10mm Double-WallPolycarbonate

1.89 0.53


1.60 0.63


1.54 0.65


1.43 0.70

Single-Pane Glass,

3mm

0.95 1.05

Poly Film 0.83 1.20

R-value: the measurement of insulating ability of the material. Thehigher the R-value, the greater the insulation value.U-value: the measurement of heat loss through the material. Thelower the U-value, the less the amount of heat that is escaping.*Independent testing,Solexx with caulked utes.

Table 4. Heat Loss and Insulating Ability of Greenhouse Coverings

Source: Adaptive Plastics, Inc.


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where:

F = perimeter heat loss factor (Btu/hour per lin-ear foot per F): Uninsulated 0.8, Insulated 0.4

P = greenhouse perimeter (in feet)

For our example, the perimeter is the distancearound the outside of the greenhouse on theground or (2*12 feet) (the ends) + (2*24 feet)(the sides) = 72 feet. Te perimeter heat loss fac-

tor for uninsulated greenhouses is 0.8, so theheat loss is Q

p= 0.8 * 72 * or 57.6. Insu-

lating around the perimeter of the greenhousewith two inches of rigid foam will reduce theperimeter heat loss by half.

Total Estimated Heat Lossas a Function of TemperatureDifference

Heat loss from all sources is added together, rep-

resenting a function of indoor air temperatureminus outdoor air temperature. Table 5shows totalheat loss estimates for two coverings, two levels ofperimeter insulation, and two levels of air inltra-tion, based on an indoor air temperature of 32oF.

Comparing these gures shows that the worstheat loss from conduction occurs throughthe hoop house covering. Even with doubleglazing, the heat loss is16 times the inltrationheat loss, and ve timesthe perimeter heat loss.

Heat BalanceIf we subtract the out-door air temperatureevery hour in the MYdata from 32o F, wecan see the total heatloss for that outdoorair temperature fromthe above relationships.

We are not concerned

with temperatures above32o F because the green-house will not freeze.Te key assumption isthat if the outdoor airtemperature is not toofar below 32o F andthere is a positive heatbalance in those hours(that is, gain is larger

than loss), then the greenhouse willnot freeze. By taking the daily solar gain andsubtracting heat loss, we can estimate how manydays we can keep from freezing based on heatbalance. We then can evaluate differentmeasures to improve insulation, improve solartransmittance, and decrease inltration.

Weather in Butte, MontanaAccording to MY data for Butte, Montana,there are 3,229 hours when the outdoor airtemperature is lower than 32oF. As Figure 5illustrates, it typically freezes in almost everymonth in Butte.

Base Case (Ineffi cient) Change Case (Effi cient)

Single-Wall Polyethylene Twin-Wall Solexx

Qc= 729T, SHGC = .90 Q

c= 291.6 T, SHGC = .75

No Perimeter Insulation Perimeter Insulation

Qp= 57.6 T Q

p= 28.8 T

Two air changes per hour One air change per hour

Qi= 36.9T Qi= 18.5T

Table 5. Total Heat Loss

Source: NCAT

HourlyOutdoorTemperature ButteMT

40

20

0

20

40

60

80

100

1/1

1/8

1/15

1/23

1/30

2/6

2/14

2/21

2/28

3/8

3/15

3/22

3/30

4/6

4/13

4/21

4/28

5/5

5/13

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11/19

11/27

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12/11

12/19

12/26

DegreesF

32F

Figure 5. Outdoor Air Temperature for Butte, Montana (o F)

Source: NCAT

Growing Season


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climate such as Butte. If we look at MY3

data and ignore all of the hours when the

outside temperature is greater than 32o F,

and then look at when the solar gain minus

the losses is positive, we can see that the

greenhouse has a better chance of protect-

ing the plants for a longer growing season

(see Figure 6).

he growing season in Butte is whenthe hourly temperature does not fall below 32o Ftypically July 1 through September 1, or abouteight weeks. When the weather forecast predictscold temperatures, gardeners in Butte often covertheir gardens overnight.

Constructing an inexpensive hoop house willobviously extend the growing season in a cold

BaseCaseSolarGains Losses

600000

500000

400000

300000

200000

100000

0

100000

200000

300000

400000

1/1

1/9

1/17

1/25

2/2

2/10

2/18

2/26

3/6

3/14

3/22

3/30

4/7

4/15

4/23

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5/17

5/25

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6/10

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6/26

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10/16

10/24

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11/9

11/17

11/25

12/3

12/11

12/19

12/27

BaseCase

Greenhouse

Season

Figure 6. Base Case Solar Gains Minus Losses

Source: NCAT


13/16


twin-wall glazing, insulating the perimeter, andreducing air leakage (see Figure 7).

Te growing season can be further extendedby building a more robust greenhouse by using

Figure 7. Solar Gain Minus Heat Loss for Effi cient Case Greenhouse

SolarGain HeatLossforEfficientCaseGreenhouse

200000

100000

0

100000

200000

300000

400000

1/1

1/9

1/17

1/25

2/2

2/10

2/18

2/26

3/6

3/14

3/22

3/30

4/7

4/15

4/23

5/1

5/9

5/17

5/25

6/2

6/10

6/18

6/26

7/4

7/12

7/20

7/28

8/5

8/13

8/21

8/29

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9/30

10/8

10/16

10/24

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11/9

11/17

11/25

12/3

12/11

12/19

12/27

EfficientCase

Growing

Season

Source: NCAT


14/16


warmer at night. Daytime solar gain can createoverheating problems even in colder seasons, andif this heat can be absorbed by thermal mass, themass can act as an energy battery and releasethe heat to the greenhouse at night. Termalmass concepts have been understood and used inbuildings for centuries, but good mathematicaltreatments of them do not exist. Te process istoo complex with too many variables to treat sim-ply with computer models. NCA plans a futurestudy of passive heat storage in greenhouses todetermine whether a numerical model of how

to predict temperature timing changes in green-houses could be developed.

Season-extending greenhouses in cold climatescannot rely on water-based systems for heat stor-age unless they are designed for freeze protectionin the winter months. Freeze protection canbe time consuming and expensivewaterfor the greenhouse must be brought into thegreenhouse from below the frost line with a frost-free hydrant.

Heating balance for January 1in Butte, Montana

Te solar gain in Butte, Montana, on January 1was charted above. Table 6shows heat loss for thegreenhouse on the same day. As the table shows,it would take two and a half times the solar heatgain to keep the greenhouse from freezing.

Our analysis ignores the thermal storage capac-ity of the soil, moisture, and tools and equipmentin the greenhouse. Tese materials will act as a

sort of thermal battery, helping to smooth out thejagged temperature lines in the charts above.

Heat Storage (Thermal Mass)Both the materials inside the greenhouse andthe soil have thermal mass. Tis thermal massgets warm when the sun shines on it and coolsoff after the sun goes down. Termal mass inthe greenhouse serves to moderate temperatures,keeping the greenhouse cooler in the daytime and

Table 6. Heat Loss for January 1 in Butte, Montana

Source: NCAT


15/16


extension technique, growers can determinewhich crops would be best suited to the result-ing growing season. Although it is not feasi-ble to extend the greenhouse season to year-around operation without supplementalheating, an inexpensive hoop house with addi-tional insulation and inltration control hasthe potential to extend the typical growingseason by more than three times the typical

cold-climate growing season.

ConclusionGrowing in cold climates is challenging,

but season-extension techniques can helpovercome these challenges and allow a lon-

ger growing season. ypical Meteorological

Year data can help eva luate different green-house designs and model solar heat gain and

heat loss from specic designs. By accurately

estimating the impact of a particular season

References

Adaptive Materials, Inc. 2011. Solexx win-Wall Green-houses and Greenhouse Covering. http://solexx.com/greenhouse_coverings.html

Bellows, Barbara. 2008. Solar Greenhouses. ARANational Sustainable Agriculture Information Service.Publication IP142. www.attra.ncat.org

Anon (eds). 1978. Joseph Orrs Fabulous Mud Heat-Stor-age Solar Greenhouse. Mother Earth News. May-June.http://www.motherearthnews.com/Modern-Homestead-ing/1978-05-01/Low-Cost-Solar-Greenhouse.aspx

National Renewable Energy Laboratory (NREL). 2008.Photovoltaic Solar Resource: Flat Plate ilted South atLatitude. U.S. Department of Energy. www.nrel.gov/gis/solar.html

National Renewable Energy Laboratory. 2005. NationalSolar Radiation Data Base: 1991-2005 Update: ypical

Meteorological Year 3. U.S. Department of Energy. http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.html

University of Georgia, No date. Greenhouse Coverings.University of Georgia, College of Agricultural and Environ-mental Sciences. www.caes.uga.edu/departments/bae/extension/handbook/documents/coverings.pdf

U.S. National Aboretum. 1990. USDA plant zonehardiness zone description. Excerpted from: USDAMiscellaneous Publication No. 1475, January 1990. U.S.Department of Agriculture. www.usna.usda.gov/Hardzone/index.html#intro

Wang, Yong-Wei and Xi-Feng Liang. 2006. Performance ofunderground heat storage system in a double-lm-coveredgreenhouse. Journal of Zhejiang University Science. March.www.ncbi.nlm.nih.gov/pmc/articles/PMC1447511

Wilcox, S. and W. Marion. 2008. Users Manual for MY3Data Sets, NREL/P-581-43156. April. National Renew-able Energy Laboratory, Golden, CO.

Wisconsin Focus on Energy. 2008. ropical Gardens Sees

More Greens. Fact Sheet BP-3182-1008. Wisconsin Focus

on Energy. www.focusonenergy.com/les/document_

management_system/business_programs/tropicalgardens

greenhouse_casestudy.pdf

Further Resources

WebsitesCommunity Garden Guide: Season Extension Hoop

Houses by USDA National Resources Conservation Service

www.plant-materials.nrcs.usda.gov/pubs/mipmctn5923.pdf

Tis useful publication discusses the advantages anddisadvantages of hoop houses, hoop house management,and recommended plants.

Frost Protection and Extending the Growing Season

by Colorado State University Extension, ColoradoMaster Gardener Program. www.ext.colostate.edu/mg/

gardennotes/722.html

Tis article discusses different types of coverings to achievefrost protection. It also provides a list of resources for furtherinvestigation.

Growing Vegetables in a Hobby Greenhouse

by Colorado State University Extension, Colorado

Master Gardener Program. www.ext.colostate.edu/mg/

gardennotes/723.html#solar

Tis article discusses season extension, passive solar green-houses, and cold- and warm-season vegetables. It alsoprovides a list of resources for further investigation.

Hightunnels.org

www.hightunnels.org

Tis website is staffed by researchers, Extension specialists,professors, growers, and others collaborating to share experi-ences and knowledge about high tunnels. Te website providesmany useful resources to growers and educators.
http://solexx.com/greenhouse_coverings.htmlhttp://solexx.com/greenhouse_coverings.htmlhttp://www.motherearthnews.com/Modern-Homesteading/1978-05-01/Low-Cost-Solar-Greenhouse.aspxhttp://www.motherearthnews.com/Modern-Homesteading/1978-05-01/Low-Cost-Solar-Greenhouse.aspxhttp://www.nrel.gov/gis/solar.htmlhttp://www.nrel.gov/gis/solar.htmlhttp://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.htmlhttp://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.htmlhttp://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.htmlhttp://www.caes.uga.edu/departments/bae/extension/handbook/documents/coverings.pdfhttp://www.caes.uga.edu/departments/bae/extension/handbook/documents/coverings.pdfhttp://www.caes.uga.edu/departments/bae/extension/handbook/documents/coverings.pdfhttp://www.usna.usda.gov/Hardzone/index.html#introhttp://www.usna.usda.gov/Hardzone/index.html#introhttp://www.focusonenergy.com/files/document_management_system/business_programs/tropicalgardensgreenhouse_casestudy.pdfhttp://www.focusonenergy.com/files/document_management_system/business_programs/tropicalgardensgreenhouse_casestudy.pdfhttp://www.focusonenergy.com/files/document_management_system/business_programs/tropicalgardensgreenhouse_casestudy.pdfhttp://www.focusonenergy.com/files/document_management_system/business_programs/tropicalgardensgreenhouse_casestudy.pdfhttp://www.ext.colostate.edu/mg/gardennotes/722.htmlhttp://www.ext.colostate.edu/mg/gardennotes/722.htmlhttp://www.ext.colostate.edu/mg/gardennotes/723.htmlhttp://www.ext.colostate.edu/mg/gardennotes/723.htmlhttp://www.ext.colostate.edu/mg/gardennotes/723.htmlhttp://www.ext.colostate.edu/mg/gardennotes/722.htmlhttp://www.focusonenergy.com/files/document_management_system/business_programs/tropicalgardensgreenhouse_casestudy.pdfhttp://www.usna.usda.gov/Hardzone/index.html#introhttp://www.nrel.gov/gis/solar.htmlhttp://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.htmlhttp://www.caes.uga.edu/departments/bae/extension/handbook/documents/coverings.pdfhttp://www.nrel.gov/gis/solar.htmlhttp://www.motherearthnews.com/Modern-Homesteading/1978-05-01/Low-Cost-Solar-Greenhouse.aspxhttp://solexx.com/greenhouse_coverings.html


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Page 16 ATTRA

Sustainable Season Extension: Considerationsfor Design

By David Ryan, PE, NCAT Energy Engineer and

Tammy Hinman, NCAT Horticulture Specialist

Published September 2011

NCAT

Rich Myers, EditorAmy Smith, Production

This publication is available on the Web at:www.attra.ncat.org

IP416

Slot 410

Version 091911

high tunnels, including construction and maintenance,the cropping environment, and heating.

Solar Gardening.1994. By Leandre Poisson and GretchenPoisson. Chelsea Green Publishing, White River Jct., V. Tis book shows how to increase efforts of the sun during the

coldest months of the year and how to protect tender plantsfrom the intensity of the scorching sun during the hottestmonths through the use of solar mini-greenhouses. Te book

includes instructions for building a variety of solar appliancesplus descriptions of more than 90 different crops, with chartsshowing when to plant and harvest each. Te result is ayear-round harvest even from a small garden.

Walking to Spring: Using High Tunnels to Grow Produce52 Weeks a Year.Alison and Paul Wiediger. Available from:Au Naturel Farm3298 Fairview Church RoadSmiths Grove, KY 42171An ordering form is available at http://aunaturelfarm.homestead.com/bookorderform.html

Tis book is based on the authors experience growing in hightunnels. It discusses site selection, construction, methods,marketing, and in-depth growing tips for crops. It alsoincludes useful charts and forms for planning, production,and marketing as well as an extensive resource list.

Individual Greenhouse Energy Conservation Checklist(adapted) By John W. Bartok Jr., University of Connecticut;A.J. Both, Rutgers University; and Paul Fisher, University ofNew Hampshirewww.hrt.msu.edu/Energy/pdf/Greenhouse%20Energy%20Conservation%20Checklist.pdf Tis checklist helps calculate the heat loss in greenhouses based

on design features.

Sunspace and Solar Greenhouse Design and Constructionby Build It Solar. http://www.builditsolar.com/Projects/Sunspace/sunspaces.htm#Storage Tis website identies numerous resources related to green-

house design and construction.

PublicationsASHRAE Handbook of Fundamentals,2009.By the American Society of Heating, Refrigeratingand Air-Conditioning Engineers, Inc., Atlanta, GA.Available from the author:

ASHRAE1791 ullie CircleAtlanta, GA 30329800-527-4723Fax [email protected] subscriptions also are available at www.ashrae.org/publications/page/2298. Te ASHRAE Handbooks are the design standard for control

of built environments with volumes on Systems and Equip-ment, HVAC Applications, Refrigeration, and Fundamentals.

Building Your Own Greenhouse. 1997. By Mark Freeman.Stackpole Books, Mechanicsburg, PA. Available from:Stackpole Books5067 Ritter Rd.Mechanicsburg, PA 17055800-732-3669 A guide to designing and constructing cold frames,

free-standing greenhouses, and attached solar greenhouses.

High Tunnel Production Manual.2003. By Bill Lamontand Mike Orzolek. Pennsylvania State University.Available from:

Dr. Bill LamontDepartment of Horticulture206 yson BuildingPenn State UniversityUniversity Park, PA 16802http://extension.psu.edu/plasticulture/technologies/high-tunnels/high-tunnel-manual Tis manual provides 18 chapters of useful information on
http://aunaturelfarm.homestead.com/bookorderform.htmlhttp://aunaturelfarm.homestead.com/bookorderform.htmlhttp://www.hrt.msu.edu/Energy/pdf/Greenhouse%20Energy%20Conservation%20Checklist.pdfhttp://www.hrt.msu.edu/Energy/pdf/Greenhouse%20Energy%20Conservation%20Checklist.pdfhttp://www.hrt.msu.edu/Energy/pdf/Greenhouse%20Energy%20Conservation%20Checklist.pdfhttp://www.builditsolar.com/Projects/Sunspace/sunspaces.htm#Storagehttp://www.builditsolar.com/Projects/Sunspace/sunspaces.htm#Storagehttp://www.ashrae.org/publications/page/2298http://www.ashrae.org/publications/page/2298http://www.ashrae.org/publications/page/2298http://extension.psu.edu/plasticulture/technologies/high-tunnels/high-tunnel-manualhttp://extension.psu.edu/plasticulture/technologies/high-tunnels/high-tunnel-manualhttp://extension.psu.edu/plasticulture/technologies/high-tunnels/high-tunnel-manualhttp://aunaturelfarm.homestead.com/bookorderform.htmlhttp://extension.psu.edu/plasticulture/technologies/high-tunnels/high-tunnel-manualhttp://www.ashrae.org/publications/page/2298http://www.builditsolar.com/Projects/Sunspace/sunspaces.htm#Storagehttp://www.hrt.msu.edu/Energy/pdf/Greenhouse%20Energy%20Conservation%20Checklist.pdf

Documents

Sustainable Season Extension for Gardening - Considerations for Design; Gardening Guidebook